Hot rolled steel sheet and method for manufacturing same

Abstract

A hot rolled steel sheet according to an aspect of the present invention has a predetermined chemical composition, and a metallographic structure in which martensite is the major structure, wherein an average length of grains in an L direction parallel to a rolling direction is 0.2 μm or more and 5.0 μm or less, an average length of grains in a C direction parallel to a transverse direction is 0.1 μm or more and 5.0 μm or less, the ratio of the average length in the C direction to the average length in the L direction is 0.2 or more and 5.0 or less, and the tensile strength is 1180 MPa or more.

What is claimed is: 
     
       1. A hot rolled steel sheet comprising, as a chemical composition, by, mass %:
 C: 0.06% or more and 0.20% or less; 
 Si: 1.0% or less; 
 Mn: more than 1.5% and 3.5% or less; 
 P: 0.040% or less; 
 S: 0.004% or less; 
 Al: 0.10% or less; 
 N: 0.004% or less; 
 Ti: 0.04% or more and 0.20% or less; 
 Nb: 0% or more and 0.04% or less; 
 Mo: 0% or more and 1.0% or less; 
 Cu: 0% or more and 0.5% or less; 
 Ni: 0% or more and 0.5% or less; and 
 a remainder consisting of Fe and impurities, 
 wherein a metallographic structure at a sheet thickness ¼ depth position and a sheet width center position of the hot rolled steel sheet consists of 90 vol % or more martensite and 0 vol % or more and 10 vol % or less of a residual structure, the residual structure is one or both of bainite and ferrite, 
 an average length of grains in an L direction, which is a direction parallel to a rolling direction, is 0.2 μm or more and 5.0 μm or less, 
 an average length of grains in a C direction, which is a direction parallel to a transverse direction, is 0.1 μm or more and 5.0 μm or less, 
 a ratio between an L direction grain length, which is the average length of the grains in the L direction, and a C direction grain length, which is the average length of the grains in the C direction, is 0.2≤C direction grain length/L direction grain length ≤5.0, and 
 a tensile strength is 1180 MPa or more. 
 
     
     
       2. The hot rolled steel sheet according to  claim 1 ,
 wherein, in the metallographic structure at the sheet thickness ¼ depth position and the sheet width center position, when an average length of prior austenite grains in the L direction is referred to as an L direction prior γ grain length and an average length of prior austenite grains in the C direction is referred to as a C direction prior γ grain length, a ratio between the L, direction prior γ grain length and the C direction prior γ grain length is 0.03≤C direction prior γ grain length/L direction prior γ grain length ≤0.40. 
 
     
     
       3. The hot rolled steel sheet according to  claim 1 , further comprising, in the chemical composition, by mass %:
 one or two or more selected from the group consisting of 
 Nb: 0.01% or more and 0.04% or less, 
 Mo: 0.01% or more and 1.0% or less, 
 Cu: 0.01% or more and 0.5% or less, and 
 Ni: 0.01% or more and 0.5% or less. 
 
     
     
       4. A method for manufacturing the hot rolled steel sheet according to  claim 1 , comprising steps (a) to (d) of:
 (a) a heating step of heating a steel material having the chemical composition according to  claim 1  to 1200° C. or higher and 1350° C. or lower; 
 (b) a finish rolling step of performing rolling by continuously passing the steel material after being heated through a plurality of rolling stands, a finish rolling start temperature being set to 800° C. or higher, the rolling being performed so as to cause an A value defined by Formula (1) in each of the rolling stands to satisfy 0.05≤A≤23.0, an interpass time between the rolling stands being set to 0.2 seconds or longer and 10.0 seconds or shorter, a final rolling stand output side temperature being set to 800° C. or higher and 950° C. or lower, a cumulative rolling reduction in 800° C. or higher and 950° C. or lower being set to 70% or higher; 
 (c) a cooling step of starting cooling within 10.0 seconds after the finish rolling and performing cooling at an average cooling rate equal to or higher than a martensite generation critical cooling rate V by ° C./s determined by Formulas (2) and (3); and 
 (d) a winding step of performing winding at a winding temperature of 300° C. or lower after the cooling, 
 
       
         
           
             
               
                 
                   
                     [ 
                     
                       Formula 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       1 
                     
                     ] 
                   
                 
                 
                   
                       
                   
                 
               
               
                 
                   
                     A 
                     = 
                     
                       
                         
                           2 
                           ⁢ 
                           π 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           n 
                         
                         
                           6 
                           ⁢ 
                           0 
                           ⁢ 
                           
                             rH 
                           
                         
                       
                       ⁢ 
                       
                         ln 
                         ⁡ 
                         
                           ( 
                           
                             1 
                             
                               1 
                               - 
                               r 
                             
                           
                           ) 
                         
                       
                     
                   
                 
                 
                   
                     ( 
                     1 
                     ) 
                   
                 
               
               
                 
                   
                     V 
                     = 
                     
                       1 
                       ⁢ 
                       
                         0 
                         
                           2.94 
                           - 
                           
                             0.75 
                             ⁢ 
                             
                               ( 
                               
                                 β 
                                 - 
                                 1 
                               
                               ) 
                             
                           
                         
                       
                     
                   
                 
                 
                   
                     ( 
                     2 
                     ) 
                   
                 
               
               
                 
                   
                     β 
                     = 
                     
                       
                         
                           2 
                           . 
                           7 
                         
                         × 
                         C 
                       
                       + 
                       
                         0.4 
                         × 
                         Si 
                       
                       + 
                       Mn 
                       + 
                       
                         0.45 
                         × 
                         Ni 
                       
                       + 
                       Mo 
                     
                   
                 
                 
                   
                     ( 
                     3 
                     ) 
                   
                 
               
             
           
         
         where n in Formula (1) is a roll rotation speed by rpm, r is a rolling reduction by %, H is a rolling input side sheet thickness by mm, C, Si, Mn, Ni, and Mo in Formula (3) are amounts of the corresponding elements by mass %, and 0 mass % is substituted into terms Ni and Mo in a case where Ni and Mo are not contained.